SPI read buffer returns incorrect values #33

With more than one read or duplex operation, the read buffer is filled with only the value of the last operation requested.
Caused by line
return [transfer.to_read_bytes() for t in transfers if t.has_read_buf]
in spi.py; suggest it be changed to
return [t.to_read_bytes() for t in transfers if t.has_read_buf]